The serial killer Peter Sutcliffe, who was given 20 life sentences in 1981 for murdering 13 women – he was known as the Yorkshire Ripper because he mutilated his victims' bodies with a sharpened screwdriver – has applied to the High Court for a ruling on how much longer he must serve in jail.

The judge at his trial recommended he serve a minimum of 30 years but did not set a formal tariff. Sutcliffe, who is now 63, wants to know how long he will have to remain behind bars before being considered for parole.

The original 30 years is up next year. News of his application to the court has outraged the survivors and families of those killed.

Why did the original judge not set a minimum sentence?

Because it was not compulsory to do so until the law changed in 2003. Before then the judge made a recommendation, which was amended or endorsed by the Lord Chief Justice (who in Sutcliffe's case recommended the minimum be increased to 35 years) and could be altered again by the Home Secretary.

But the law was altered by the Human Rights Act, which requires that judges, not politicians, have the final decision. When in 1994 the then Home Secretary, Michael Howard tried to extend the term to be served by the killers of Jamie Bulger, the courts ruled his act was illegal. Now only judges have a say.

So Sutcliffe has obtained legal aid and instructed his lawyers to apply to the High Court for a formal tariff to be set in his case. The hearing now being held in London is to decide what form the tariff-setting hearing should take, and what evidence should be admitted.

Sutcliffe wants a 30-year tariff; the Justice Secretary, Jack Straw, along with the victims' families, who have submitted "victim impact statements", have applied for life to mean life.

How will the court go about making its decision?

The guidance is fairly clear. A "whole-life tariff" is recommended where the offender has murdered two or more people or where a murder has involved a substantial degree of premeditation or planning, if the victim was abducted or there was a sexual or sadistic element to the killing.

So what makes Sutcliffe's case more complex?

At the original trial the prosecution initially accepted the diagnosis of doctors that Sutcliffe suffered from paranoid schizophrenia and sought to have him placed immediately in a secure mental hospital.

However, the trial judge insisted that there should be a trial so that a jury could decide whether Sutcliffe was mad or bad.

Prosecutors then claimed that Sutcliffe had fooled psychiatrists into believing that he had heard "divine voices" telling him to kill. The jury agreed and Sutcliffe was found guilty of murder.

But within three years of being jailed Sutcliffe was transferred to Broadmoor top security psychiatric hospital after sectioned under Section 47 of the Mental Health Act. In addition what is known as a "restriction order" was made.

That means that he cannot be released from his detention under the Mental Health Act until his jail sentence has been completed.

So he gets a fixed sentence, then declares himself sane, transfers to an ordinary prison, and applies for parole?

It's more complicated than that. Before he can be transferred to an ordinary prison he has to convince a mental health tribunal – made up of a judge, consultant psychiatrist and lay member – that he is no longer suffering from a mental illness.

That tribunal sits automatically on cases like Sutcliffe's every three years – and the last one ruled that he is still mad and must remain in Broadmoor.

Things could change on that. Kevin Murray, a consultant psychiatrist at Broadmoor secure hospital, who has treated Sutcliffe since 2001, has said that it was the unanimous view of his colleagues that the Ripper was suffering from paranoid schizophrenia at the time of his crimes. But, in a report in 2006, Dr Murray concluded that Sutcliffe has now responded well to treatment and has been virtually "cured". Dr Murray said in the report three years ago Sutcliffe may be safe for release within a "low single figures" of years.

If he is sane will he get parole?

Not necessarily. To do that he has to convince the Parole Board, which is also chaired by a judge, of a number of things. He must demonstrate genuine remorse for his crimes, which Sutcliffe has so far refused to do, according to the relatives of his victims. He must show that he has made significant progress under treatment. And he must also show that he poses "little or no risk" to the public.

Sutcliffe's lawyers would argue that is indeed the case, since the doctor treating him says that his delusions can be controlled with medication. They would also point out that Sutcliffe is now clinically obese, has diabetes, is blind in one eye and only has partial sight in the other – and is therefore not much danger to anyone.

But is that argument really convincing?

Not everyone is convinced. "He may be fine if he takes his medication," one criminologist said yesterday. "But how do you make sure he takes his medication once he's out of hospital?" For the first nine years that he was in Broadmoor Sutcliffe refused treatment until the Mental Health Commission in 1993 ruled it should be given forcibly.

"People with schizophrenia often relapse, with or without medication, and behave unpredictably," one prison doctor said. "No one can know how a man who has been in captivity for almost 30 years might react, in the real world, on meeting a young woman."

Despite all the tabloid stories to the contrary, getting clearance from the Parole Board is no easy matter. In 2006/7, the last year for which figures are available, some 85 per cent of applications to the Parole Board by people on indeterminate sentences like Sutcliffe were in fact refused.

What other options does Sutcliffe have open to him?

The judge at the current hearing, Mr Justice Mitting, has been told by the killer's psychiatrist, Dr Murray, that in his view Sutcliffe has been wrongly convicted of murder. Because his colleagues at Broadmoor now unanimously agree that Sutcliffe was mentally ill at the time of the killings he should have been allowed to "diminished responsibility" at the time of his trial and should have been found guilty of manslaughter.

The judge has said a psychiatrist's report to that effect could in law form the basis for Sutcliffe to appeal against his murder convictions. Were that to happen it would trigger even greater public fury.

Is it time to set Peter Sutcliffe free?

Yes...

* The rules are that if you have killed more than once, with pre-meditation or with sexual or sadistic motives, then life will mean life

* He is yet to convince a mental health tribunal that he is not still mad

* Some 85 per cent of applications for parole from people with indeterminate life sentences are refused

No...

* Next year he will have served the 30-year sentence his trial judge recommended as a minimum

* His psychiatrist says that with medication his delusions can be controlled

* At 63 he is fat, diabetic and almost blind. That means he is no longer a danger to the public